Spectrometry (or spectrophotometry) is an optical method for measuring the spectral response of an article to incident light including a plurality of wavelengths. This response may be based on reflectance (the method known as “reflectometry”), polarization change (the method known as “polarimetry” or “ellipsometry”), grating diffraction efficiency (the method known as “scatterometry”), and others.
In the spectrometric methods, one of the most important elements is a light source for article illumination, which produces a light beam of desired geometry, intensity and spectral range; and a light detection system that transforms light returned from the illuminated article into an electrical signal to be further digitized and processed with appropriate electrical and computing means.
In the spectrometric methods, various optical components are used for implementing a chosen optical scheme. A proper optical design is selected being aimed at keeping the brightness of a light source, i.e., providing attenuation of the light intensity by optical components as little as possible, in order to reach maximum signal-to-noise ratio (SNR) of the output measured signal. This SNR depends mainly on the sensitivity and noise of a detector, brightness of a light source, attenuation of an optical scheme, and the article properties as well. The higher SNR, the more precise measurements may be achieved.
A light detector typically has a limited range of signal to be accurately measured. A low level of the measured signal is limited by its intrinsic noise and shot noise of illumination light, while a high level of the measured signal is limited by saturation, i.e., the level of signal above which the light detector is no more sensitive to incident light. A ratio between the saturation and noise levels is a “dynamic range” of the light detector.
A high dynamic range is typically needed to enable accurate measurements with a required SNR in a wide range of varying properties of the measured article. For example, if an article under measurements is a silicon wafer with a multiple layer coating, and reflectance of the wafer is measured, a reflectance coefficient of such an article may vary from single percents at a certain wavelength range to about 100% at another wavelength range. Therefore, the required dynamic range of a light detector is to be large enough in order to provide a desirably high SNR in the entire range of measured parameters of the article.